Actually it's the other way around. The designer of the airfoil comes up with (emperically usually) a point known as the center of lift. The aircraft designer uses that information based on the mission of the aircraft.
Assume that we are talking about conventional aircraft here (big wing in front, little wing in back) For most GA aircraft, the idea is to keep the CG of the aicraft ahead of the center of lift (nose heavy). This creates a pitching moment that pushes the nose of the aircraft towards the ground. The designer then sizes the tail accordingly to have enough force to overcome that pitching moment. IF he gets it right, the aircraft will be stable. If the center of lift and the center of gravity are coincident, the tail plane has to apply zero force to keep the aircraft level... however, this is akin to balancing a ball bearing on an up-side-down bowl or other convex surface.... The nose heavy condition will make the aircraft, in general, more stable, slower in cruise (due to more induced drag because the wing is carring more effective weight) and make landing distances longer. Stall speed will also usually increase, but with more of a tendancy to mush (again, in general). Controls will feel heavier as well. Yes, there is such as thing as too nose heavy. This is where the tail plane simply cannot apply the needed down force required to overcome the pitching moment created by the wing. Very bad when you are 3 feet off the runway and slow... The more towards the tail heavy end you go, up to the coincindence point, the more touchy (sometimes that can be fun) the aircraft becomes, and the faster it will fly (less induced drag). Stall speed will likely decrease, but it may make recovery longer. Control feel gets lighter as well. If you go too far aft, well lets just say that's not a good idea. Anyone that has flown a tail heavy aircraft, real, model or otherwise, knows what I'm talking about. I know that I have grossly generalized some information, but there are many good books on this subject alone if you want/need to get into the nitty-gritty. Hope this helps, Matt --- "Dene Collett (SA)" <[email protected]> wrote: > Hi guys > Thanks to everyone that tried to stear me in the right > direction regarding > CG location. I have really very limited knowledge of > aerodynamics and have a > few questions of how the CG is determined. The way I see it is > this : After > the "designer" has finished designing his aerofoil, he does > some more > calculations or some testing (wind tunnel?) to determine the > CG range that > the aerofoil is happy at regardless of what plane it will > ultimately be used > on. Then again I suppose that would depend on the amoumt of > load the wing is > expected to carry and the speed it is expected to carry it at. > So if I stick to an ultimate aft limit of 30% MAC I should be > OK??? By the > way, what are the published figures that everyone talks about. > I only have > the remains of a very old manual and I guess that bit of info > is missing. if > it wasn't for two friends who had built three KRs between > them, I might > never have got this far. > Mark, the tube is for the mains and yes it is a retract I'm > afraid (all > three wheels). I am basing the system on a design by Stelio > Frati called the > "Pinguino". I figured it took me nine years to get this far, I > might as well > try to produce something really unusual as well as not make it > any heavier > than the fixed gear. > Thanks again guys. > Cheers > Dene Collett > KR2S-RT builder > Port Elizabeth > South Africa > mailto: [email protected] > P.S: checkout www.whisperaircraft.com > > > > _______________________________________ > Search the KRnet Archives at > http://www.maddyhome.com/krsrch/index.jsp > to UNsubscribe from KRnet, send a message to > [email protected] > please see other KRnet info at http://www.krnet.org/info.html > ===== ------------------------------------------------- Matthew Elder Orangeburg, SC http://www.infinigral.com/melder
